Conventionally, the traveling speed of a hydraulic excavator is controlled by supplying hydraulic pressure from a hydraulic pump 51 to left and right traveling motors motor 54a, 54b via a swivel joint 53, after controlling the hydraulic pressure by left and right travel operation valves 52a, 52b, as shown in FIG. 3. When a traveling speed changeover switch 55 is set to a first stage travelling speed, the solenoid of a control pressure changeover valve 56 is de-energized so that the control pressure changeover valve 56 is set to a position a. Therefore, pilot cylinders 59a, 59b of the traveling speed changeover valves 58a, 58b for the left and right traveling motors 54a, 54b are connected to a tank 66 via the swivel joint 53 and the control pressure changeover valve 56, so that the traveling speed changeover valves 58a, 58b also are set to their position a. Consequently, traveling motor capacity control cylinders 60a, 60b are connected to the tank 66, so that the capacities of the left and right traveling motors 54a, 54b are increased to a value corresponding to the first stage of the traveling speed.
Subsequently, a traveling operation lever (not shown) is operated to set the traveling operation valves 52a, 52b to their position a, so that the hydraulic oil discharged from the hydraulic pump 51 is supplied to the left and right traveling motors 54a, 54b through associated driving lines 61a, 61b, and is discharged from the left and right traveling motors 54a, 54b into discharge lines 62a, 62b. In this state, counterbalance valves 63a, 63b have been set to their position a by the operating hydraulic pressure existing in the driving lines 61a, 61b, so that the oil discharged to the discharge lines 62a, 62b is returned to the tank 66 via a return line (not shown, past the counterbalance valves 63a, 63b, swivel joint 53 and the running operation valves 52a, 52b. Therefore, the left and right traveling motors 54a, 54b, which have been set to first stage traveling speed, are operated at a speed corresponding to the amount of operation of the above-mentioned traveling operation lever, whereby the excavator travels forward or backward at the corresponding speed. The positions a of the travel operation valves 52a, 52b are assumed to correspond to the forward traveling of the excavator. Thus, the positions b of the same correspond to backward traveling. The operation for the backward traveling is not described because it is substantially the same as that for the forward traveling. Numeral 64 designates a load sensing valve which has a first pilot chamber 64b for receiving oil discharged from the hydraulic pump 51 and a second pilot chamber 64c for receiving the higher one of the load pressures existing in the driving lines 61a, 61b of the traveling motors 54a, 54b, via the traveling operation valves 52a, 52b and check valves 65a, 65b. The discharge pressure of the hydraulic pump 51 is so reduced as to maintain a balance of force and counter force produced by the pressures in the first and second pilot chambers 64b and 64c and the force produced by a spring 64a, thereby controlling the capacity control cylinder 51a of the hydraulic pump 51. That is, a pressure differential corresponding to the force of the spring 64a always exists across each travel operation valve 52a, 52b, so that the rate of discharge of oil from the hydraulic pump 51 is increased and decreased in accordance with the amounts of operation of the travel operation valves 52a, 52b.
For attaining the second speed stage, the traveling speed changeover switch 55 is set to the second stage traveling speed so that the solenoid 56a of the control pressure changeover valve 56 is energized to set the control pressure changeover valve 56 to the position b. As a result, the traveling speed changeover valves 58a, 58b of the left and right traveling motors 54a, 54b are set to their position b. Consequently, driving hydraulic pressures in the left and right traveling motors 54a, 54b are introduced into the traveling motor capacity control cylinders 60a, 60b, whereby the capacity of the left and right traveling motors 54a, 54b is decreased to establish the second speed stage. The effect produced by the operation of the traveling operation valves 52a, 52b is not described because it is materially the same as that in the traveling at the first speed stage.
In this conventional system, the speed of the traveling motors 54a, 54b are controllable only in two stages. This causes an excessively abrupt change in the traveling speed when the speed stage is switched from one to the other to attain the traveling condition which is optimum for the instant state of work and the road condition. In addition, if one of the high- and low-speed stages is set to an optimum level, the speed obtained with the other speed stage is often too high or too low, which makes it difficult for the operator to select the optimum speed stage according to the state of work and road conditions. In general, the conventional traveling system is so designed that the discharge rate from the hydraulic pump 51 is maximized when the excavator is traveling. Therefore, if the working machine is operated during traveling, the excavator may be suddenly decelerated to oscillate even when the amount of operation of the working machine is very small. Thus, the conventional arrangement is not suitable for simultaneous operation of the traveling system and the working system.
In view of the above-described shortcomings of the known art, the present invention aims at providing a traveling speed changeover device for a hydraulic excavator, wherein a set force adjusting means is provided to enable adjustment of the set differential pressure so as to make it possible to change the speed of the traveling motors over a plurality of stages, thus optimizing the traveling speed for instant road conditions, while eliminating deterioration of simultaneous operability which hitherto has occurred when the working machine is operated during traveling.